The Pliocene Epoch is the period in the geologic timescale that extends from 5.3 million to 2.6 million years before present. The global average temperature in the mid-Pliocene was 2-3°C higher than today, global sea level 25 meters higher and the Northern hemisphere ice sheet ephemeral. A few years ago, however, Brierley’s team found evidence suggesting that the tropical Pacific was even warmer during the Pliocene than anyone had expected. The results were found in sediment cores drilled from the ocean floor. By analyzing the chemical properties of the sediment, researchers were able to determine the ocean’s temperature in the past. Their findings showed that a huge pool of warm water covered the vast majority of the Pacific and that the temperature gradient of sea surface was smaller than previously predicted—that is, the warm pool in the central Pacific was larger and more uniform in temperature. This warm pool is similar to the effects of the periodic El Niño phenomenon, which causes warming of the Pacific near South America. However, because the Pliocene pool was sustained, it has been dubbed a permanent El Niño.

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Surprisingly, model projections of the future do not show significant changes in the pattern of sea surface temperatures in the tropical Pacific. Many geological factors, including the location of the continents, atmospheric carbon dioxide levels, and the intensity of sunlight, were all similar to those in present day. Yet, the water off the coast of California was considerably warmer than today’s. How could two similar sets of geological inputs produce two different outcomes in temperature distribution? Researchers could not explain the discrepancy.

To resolve the discrepancy, researchers used models. The results highlighted an inconsistency between the predicted and the actual, leading to speculations about the validity of current models. Brierley explains, "They suggested some disconnect between current models of climate and observations of the past. Maybe something important was wrong with our models."

The team then set about searching for an explanation that could explain why current models were not predicting this ancient warm pool. Then, about two years ago, Dr. Kerry Emanuel, Professor of Meteorology at the Massachusetts Institute of Technology and renowned hurricane expert, gave a talk at Yale about why tropical cyclones contribute to ocean mixing. Emanuel, who had speculated about the role of tropical cyclones played in global warming in the past, inspired Brierley’s team to consider hurricanes as the missing link: "We were wondering how to make this warm pool happen. And when Kerry came down, we realized that tropical cyclones were the phenomenon we were looking for."

"Essentially, we've looked at a warm world in the past and it shows changes in the pattern of tropical sea surface temperatures. We've analyzed all the existing theories to explain this vast pool of ancient warm water, and even in combination they can't explain something as odd as this," said Dr Chris Brierley (UCL Geography), a co-author of the paper.

Brierley represented the hurricanes in the model by including additional mixing in regions with more predicted hurricanes. Such a model is more representative of the global climate state of the Pliocene. The additional mixing from the hurricanes altered the results considerably, as the new model predicted a "strong reduction of the strong temperature gradient near the equator:, i.e., the existence of the Pliocene warm pool.

Brierley explains that the results can be understood by considering the effect of hurricanes on global ocean circulation. The cold water off the coast of Ecuador has its source in the cold region in the central extra-tropical Pacific. It travels at depth to reach the Equator, avoiding the major regions of cyclone activity as it does so.

This leads to a positive feedback loop that could explain the permanent El Niño. The warm pool causes increased tropical cyclone activity in the central extra-tropical Pacific. Powerful surface winds stir the ocean, pumping warmer surface water to the bottom and drawing colder water upward. The heat pumped downwards warms the ocean currents. The ocean currents well up towards the equator, leading to warmer ocean surface temperatures in the El Niño region. These warmer surface conditions alter atmospheric circulation, causing greater hurricane activity in the central Pacific. This system, caught in a positive feedback loop, contributes to the sustained El Niño.

What does this mean for our future as the globe warms up? Could we have another permanent La Nino forming a new warm pool and stabilizing at least some of the climate effects?